This application is a 371 of PCT/FR98/00580 Mar. 23, 1998.
This invention relates to an electrolytic method for the selective recovery and recycling of silver from a nitric acid solution containing silver in the form of Ag(I), this method comprising successive steps of electro-deposition and of redissolution of the deposited silver Ag(0).
The invention is particularly applied to the selective recovery of Ag(I) by reduction to Ag metal from nitric acid solutions arising from the processing of plutonium containing waste in plants for the reprocessing of nuclear fuels and to the redissolution of the silver metal to form Ag(I) with the purpose of recycling it to an upstream point of the method.
The use of an electrolytic method to recover metallic silver from solutions or from effluents notably containing silver ions has been known for a long time.
Hence, document FR-A-0 704 663 describes a method of separating silver in the metallic state by electrolysis of hyposulfite solutions containing silver ions Ag(I), used, in particular, in photography.
This method consists of adding to the solution to be treated, apart from gelatin, a specific activating agent containing sulfur that is intended, during the electrolysis, to provide very slight cathodic deposits of silver sulfide. This document is only concerned with the extremely specific problem of electrolysis of the silver from hyposulfite baths and contains no disclosures that could be applied to the electrolysis of nitric acid solutions; furthermore, the redissolution of the deposited silver is certainly not described.
Document U.S. Pat. No. 4,226,685 relates to a method for the removal of heavy metals such as silver and cyanide ions from effluents from electrolytic plating which uses an electrolysis apparatus in a single enclosure, with no separate compartment, comprising cathodes with a high specific surface area formed from a bed of conductive particles. Both the specific treatment of nitric acid effluents and possible redissolution of the metal, for example deposited silver, are certainly not described, it only being mentioned that the cathodes on which the metals are to be found, are dismantled, removed from the apparatus, and later processed by traditional purification techniques.
In an analogous way, document FR-A-0 699 738 relates to an electrolytic xe2x80x9cdepollutionxe2x80x9d reactor for the treatment of effluents containing heavy metals that comprises a single tank in which electrodes of opposite polarity are alternately positioned, the cathodes preferably being in the form of detachable high volume electrodes with a granular filling, for example based on petroleum coke.
Once again, the processing of nitric acid solutions notably silver solutions is not touched on. The recovery of the deposited metal takes place by removing the detachable cathode and processing the granular bed in the traditional way outside of the tank, in order to recover the metal.
The problem of recovering and recycling silver from nitric acid solutions is posed in a particularly acute way in plants for the reprocessing of nuclear fuel.
In these plants, the remarkable aptitude that the Ag(II) species has for xe2x80x9cquasi-instantaneouslyxe2x80x9d dissolving plutonium dioxide in a nitric acid medium is made use of. This is described particularly in documents by J. BOURGES, C. MADIC, G. KOEHLY, M. LECOMTE xe2x80x9cDissolution du bioxyde de plutonium en milieu nitrique par l""Ag(II) xc3xa9lectrogxc3xa9nxc3xa9rxc3xa9xe2x80x9d J. of the Less Common Metals, p. 122, 303, 1968 and the document by M. LECOMTE, J. BOURGES, C. MADIC xe2x80x9cApplications du procxc3xa9dxc3xa9 de dissolution oxydante du bioxyde de plutoniumxe2x80x9d in Proceedings of the In. Conf. On Nucl. Fuel Reprocessing and Waste Management RECOD 87, Vol. 1, p. 444, 1987.
Because of this, the use of the Ag(II)/Ag(I) redox couple has become progressively more important over the last few years in reprocessing plants such as that at La Hague, as is mentioned in the document by F. J. PONCELET, M. H. MOULINEY, V. DECORBERT, M. LECOMTE xe2x80x9cIndustrial Use of Electro-generated Ag(II) for PuO2 Dissolution RECOD 94 Proceedingsxe2x80x9d, Vol. II, p. 24-28 April 1994, Londonxe2x80x94United Kingdom.
In particular, the technique of xe2x80x9coxidizingxe2x80x9d dissolution of PuO2 with Ag(II) electro-generated from silver salts and notably from AgNO3, must be used for the processing of certain plutonium containing wastes as pointed out in the document by J. BOURGES, M. LECOMTE, J. C. BROUDIC, M. MASSON, D. LALAQUE, J. P. LECOURT xe2x80x9cDxc3xa9contamination des dxc3xa9chets solides contaminxc3xa9s en xc3xa9metteurs alpha, beta, gamma, en vue de leur dxc3xa9classement en terme de stockagexe2x80x9d EUR 15 804 FR, 1994 and the document by M. H. MOULINEY, F. J. PONCELET, P. MIQUEL, V. DECORBERT, M. LECOMTE xe2x80x9cElectrogenerated Ag(II) for Recovery of PuO2 from Wastexe2x80x9d ENC 94, 286 1994, Lyon, France.
However, this introduction of silver in the form of AgNO3 in the downstream part of the cycle does pose numerous problems for the treatment of the effluents created which are sent at the end of the cycle for vitrification.
Cost constraints on the one hand and constraints linked to the quality of the vitrified waste on the other hand require that the quantity of silver introduced into the glasses be limited.
It is therefore necessary to recover the silver found in the form of Ag(I) in the concentrated nitric acid solutions arising from the reprocessing.
Among the methods for recovering the silver from nitric acid solutions contaminated with alpha emitters, a process is known, on the one hand, of reducing and precipitating the Ag(I) as Ag(0) with ascorbic acid, usually with the addition of anti-nitrous reactants hydrazine, hydroxylamine nitrate and on the other hand a method of reducing the Ag(I) to Ag(0) carried out in an electrolytic unit with a separate compartment.
These methods are described respectively in documents by E. J. WHEELWRIGHT, L. A. BRAY, J. L. RYAN xe2x80x9cApparatus and Methods for Dissolving Hazardous Waste Materials by Catalyzed Electrochemical Dissolutionxe2x80x9d WO-A-89/10981, November 1989 and by D. BLANCHARD, J. E. SUMMA, D. L. ALEXANDER, E. J. SHADE, J. D. MATHESON, T. E. BOYD, D. L. COCHRAN, E. J. WHEELWRIGHT xe2x80x9cRecovery of Silver from CEPOD Anolyte Solutionsxe2x80x9d Final Report -PNL- 10164 UC 721, September 1994.
The method of separating silver by precipitation has two disadvantages, namely the use of large quantities of reducing agents and anti-nitrous agents often undesirable and difficult to degrade, poses difficulties for the subsequent management of the effluents and the need for a solid/liquid separation to be carried out.
The electro-deposition method such as the one used in the document by WHEELWRIGHT already mentioned above, has the major disadvantage of using an apparatus with a separate compartment with a high ohmic drop, while the possibility of redissolution is neither described nor considered.
The methods described in documents U.S. Pat. No. 4,226,685 and EP-A-0 449 735 which use electrolysis equipment with a single compartment and which do not touch on the specific problem of the processing of nitric acid solutions, have the major disadvantage of including a possible redissolution step that is separate both in space and in time. This step is carried out in an off-line fashion in an installation that is independent of the electrolysis unit.
Such a disadvantage is even more of a problem in the case of processing solutions that contain radioactive elements.
Document JP-A-05 188 187 describes, according to its abstract, the recovery of metals from a nitric acid dissolution solution that arises, for example, from the dissolution of spent nuclear fuel. The method described in this document uses electrolysis at a controlled potential without the addition of anti-nitrous compounds.
Consequently, the objective of the invention among others is to correct the disadvantages of the methods of the prior art.
These objectives and others are achieved in accordance with the invention by an electrolytic method of selective recovery and recycling of the silver from an initial nitric acid solution containing it in the form of Ag(I), comprising successive steps of electro-deposition of the silver Ag(0), and then redissolution of the deposited silver Ag(0) to give a final solution of silver Ag(I), in which said electro-deposition step is carried out with a prescribed current and said successive steps of the electro-deposition of the silver, and of redissolution of the deposited silver are carried out in the same apparatus.
The method according to the invention therefore allows one to overcome two of the disadvantages inherently linked to the methods of the prior art described above.
On the one hand, it does not make use of any agents such as reducing agents that are difficult to degrade. All of the products used are in effect, products that can be easily degraded and do not any problems during subsequent treatment of the effluents. In particular, it is possible to avoid a solid/liquid separation which is difficult to carry out in an active environment.
On the other hand, the fact that the steps of recovering the silver Ag(0) and redissolving it are integrated within the same apparatus leads to an important simplification of the process, to the installations being much more compact, to a reduction in the number of pipes and other devices for the passage of fluids and solids etc.
All these advantages of the method are particularly important in an active environment.
The gain in space is accompanied by a gain in time since neither the redissolution process nor the recycling is carried out off-line on a specially designed site.
Consequently, the method of the invention is much more economic than the methods of the prior art. Furthermore the fact that the electro-deposition step is carried out at a prescribed current means that compared with methods operating at a controlled potential the electro-deposition time is limited. In effect it is the current that governs the quantity of silver deposited.